Reassortants with characteristics that make them potential vaccine candidates have been isolated from coinfection of primary tissue culture with fastidious human rotavirus (strains D, DS-1, P or ST3, representing serotypes 1, 2, 3, or 4, respectively) and a wild type bovine or rhesus rotavirus. Analysis of the genotypes of these reassortants revealed that many contained 10 genes from the animal rotavirus parent and only one gene, that which codes for the major neutralization protein, VP7, from the human rotavirus parent. Single human rotavirus gene substitution reassortants which have the human rotavirus neutralization specificity as determined by plaque reduction neutralization assay (PRNA) are available for each of the above coinfections (i.e., RRV x D, DS-1, or ST3; UK x D, DS-1, P, or ST3). These reassortants represent promising candidate live vaccine strains. Their animal rotavirus gene complement should attenuate them, but the major neutralization protein of human rotavirus should induce protective immunity. These single human rotavirus gene substitution reassortants have been adapted to growth in DBS-FRhL cells and 2 reassortants, DxRRV (6-1-1) and DS-1xRRV (240-2-1) have been prepared as vaccine lots at Flow Laboratories and are currently undergoing safety testing. Careful neutralization studies of the reassortants described above have shown that a gene product in addition to VP7 (coded by gene 8 or 9) is also involved in neutralization. This other gene product appears to be VP3, coded for by the 4th gene. Attempts are being made to isolate reassortants which derive both their 4th and 8th or 9th genes from the human rotavirus parent but the remaining 9 genes from the animal rotavirus parent.